1. Field of Invention
The invention generally relates to photovoltaic solar cells and, more particularly, to high-efficiency multijunction photovoltaic solar cells based on III-V semiconductor compounds.
2. Description of Related Art
Multijunction solar cells, based on III-V semiconductor compounds, have demonstrated high efficiencies for the generation of electricity from solar radiation. Such cells have reached 35.8% efficiency under the AMOG spectra (http://www.sharp-world.com/corporate/news/091022.html) and 43.5% (see M. A. Green et al., Progress in Photovoltaics: Research and Applications 19 (2011) 565-572) under concentrated sunlight equivalent to several hundred suns. Such efficiency and power achievements make it possible to apply this technology to the space and terrestrial energy markets. The solar cells with the highest efficiencies to date have employed three subcells having differing energy band gaps and arranged to permit each subcell to absorb a different part of the solar spectrum. Each subcell comprises a functional p-n junction and other layers, such as window and back surface field layers. These subcells are connected through tunnel junctions, with the layers either lattice matched to the underlying substrate or grown over metamorphic buffer layers.
Each subcell typically includes a window, emitter, base and back surface field (BSF) and may or may not include other layers. Those of skill in the art will also recognize that it is possible to construct subcells that do not contain all of the foregoing layers. The window and the BSF reflect minority carriers away from their interfaces with the emitter and base layers, respectively, and are well known to be critical to high efficiency carrier collection. The materials and doping levels used for windows are chosen such that the band alignment produces a large energy barrier for the minority carriers with a minimal barrier for majority carriers. This allows majority carriers to diffuse through the window, while minority carriers are reflected. It is critical that the interface between the window and the emitter be very high quality, to minimize the minority carrier surface recombination velocity. The window also typically has a higher band gap than the adjacent emitter in order to minimize its absorption of incident light.
For the top subcell, the window can be a major source of current loss. The window absorbs a fraction of the incident light in the solar spectrum that is above its band gap, and generates electron-hole pairs, or photocarriers. These photocarriers are not collected with high efficiency due to the high surface recombination velocity for minority carriers at the top of the window, and the low minority carrier diffusion lengths that are common in window materials. In subcells below the top subcell, the band gap of the window need not be as high as in the top subcell, because the top subcell will already have absorbed the higher energy photons. The window layer of lower subcells may be a source of loss if the upper subcell(s) do not absorb all light above the band gap of this window.
The intrinsic material lattice constant is defined as the lattice spacing a material would have as a free-standing crystal. When a semiconductor material has a substantially different intrinsic lattice constant than the substrate or the underlying layers on which is grown, the material will initially adopt the lattice constant of the underlying layers. The semiconductor material is strained, however, and the degree of strain is proportional to the difference in intrinsic material lattice constants between the material and the adjacent material on which it is grown.
As the thickness of such a semiconductor layer is increased, the accumulated strain increases until a critical thickness is reached, after which it becomes energetically favorable to relax and relieve strain through dislocation, i.e., departure of the atoms from their normal crystalline structures. The critical thickness depends upon many factors, including the materials involved, the substrate and/or underlying layers, growth technique and growth conditions. For a difference in intrinsic material lattice constants of about 1%, U.S. Pat. No. 4,935,384 to Wanlass et al. teaches that the critical thickness is around 15 nm. Below that critical thickness, Wanlass reports that the semiconductor layer is considered pseudomorphic, or fully strained, and the semiconductor layer holds the lattice constant of its substrate or underlying layer in the plane perpendicular to the growth direction. Typically, such a layer will have a different lattice constant in the direction of growth, with all lattice constants different from the material's intrinsic material lattice constant. The semiconductor layer is considered fully relaxed when sufficient dislocations have formed that the layer has been essentially restored to its intrinsic material lattice constants. In general, layers may be fully strained, fully relaxed, or partially strained and partially relaxed when grown on top of a substrate or layers with a substantially different lattice constant. This discussion has assumed that the materials have a cubic crystal structure in which the intrinsic material lattice constant is the same in all three crystal directions. An analogous discussion is appropriate for materials which are not cubic.
The prior art is primarily photovoltaic cells with window layers that have nominally the same intrinsic material lattice constants as the cell layers beneath them. For a given alloy system, choosing the lattice constant fixes the material composition and therefore its relevant properties such as its band-gap energy. For example, fully disordered AlxIn1-xP has substantially the same intrinsic material lattice constant as a GaAs substrate where x=0.52. This composition has an indirect band gap of 2.29 eV and a direct band gap of 2.37 eV at 300K. Strained, pseudomorphic window layers are mentioned by Wanlass et al. and King et al. (U.S. Pat. No. 7,119,271), but, as mentioned above, the teaching is that the critical thickness is 15 nm for a 1% difference in lattice constants. A thickness of 15 nm or less is too thin for practical use in many multijunction solar cells; hence, King et al. focus on window layers that are fully relaxed rather than pseudomorphic.
King et al. use a fully relaxed, high-band-gap window layer that incorporates dislocations to achieve relaxation in photovoltaic cells. While relaxation via dislocations have been claimed to improve interface quality and minimize defect transport, the greater body of work in the literature shows that dislocations are non-radiative recombination centers that degrade the quality of the material and reduce its current collection efficiency. In addition, defects at the interface of the emitter and window can increase the surface recombination velocity of the minority carriers and further degrade the solar cell efficiency. Thus, fully relaxed window layers are not ideal for high efficiency solar cells.
To improve the efficiency of high efficiency solar cells, it is desirable to maximize the band gap of the window layer of the top subcell, which typically reduces the light absorption in the window and increases the current of the solar cell, while avoiding dislocations that would be produced by relaxation.